1. Field of the Invention
The present invention relates to a paint composition for cation electrodeposition painting which composition is improved in ability (referred hereafter to as "edge covering ability") for covering the edge of a metal member to be painted.
2. Description of the Prior Art
Cation electrodeposition painting has been industrially widely utilized to accomplish painting of a variety of metal members. In the cation electrodeposition painting, the member (serving as a cathode) to be painted is dipped in a cation type aqueous paint liquid prepared by neutralizing resin having basic groups, and then direct current is passed from an anode through the paint liquid to the member to be painted, so that a paint coating film is formed on the surface of the member to be painted. Additionally, a variety of paint compositions have been also proposed and put into practical use. In this connection, Japanese Patent Publication No. 52-6306 and Japanese Patent Provisional Publication No 52-11228 disclose reaction products of amine-added compound of epoxy resin and partially capped polyisocyanate; however, they do not disclose paint composition improving the edge covering ability. Furthermore, Japanese Patent Provisional Publications Nos. 51-119727, 52-147638 and 56-151777 disclose compositions obtained by adding amine or acrylic type double bond to epoxidated compound of a polymer having carbon-carbon double bond and a low polymerization degree, for example, liquid polybutadiene; however, they fail to disclose a paint composition exhibiting excellent edge covering ability, corrosion preventing ability, flexibility, coating film smoothness and low temperature hardening characteristics.
Thus, the above-mentioned conventional paint compositions cannot meet recent requirements of rust prevention in automotive vehicles, i.e., high level edge covering ability or edge rust preventing ability. With the conventional paint compositions, although a major part of a metal member to be painted is coated with a paint coating film having a film thickness of 15-25 micron to be intended to provide sufficient rust prevention ability, the edge of the metal member is coated with only the paint coating film having not more than 3 micron thickness in Which the film thickness depends on edge corner angle, thereby accelerating production of rust. In view of this, the following countermeasures requiring troublesome treatment in addition to the electrodeposition painting have been carried out: for example, the edge of the member to be painted is bent inside to come to a position out of sight, during machining such as cutting before electrodeposition painting. Otherwise, the edge may be covered with an additional material after electrodeposition painting.